Hydrogen or hydroxyl ions

Facilitated transport membranes can be used to separate gases membrane transport is then driven by a difference in the gas partial pressure across the membrane. Metal ions can also be selectively transported across a membrane driven by a flow of hydrogen or hydroxyl ions in the other direction. This process is sometimes called coupled transport. 

In your chemical experiments you ll he working with two classes of salts normal salts (such as NaCl, Na CO, KI) which contain no free hydrogen or hydroxyl ions, and acid salts (such as NaHS04l NaHCO ) which contain replaceable hydrogen. 

HYDROLYSIS When salts are dissolved in water, the solution is not always neutral in reaction. The reason for this phenomenon is that some of the salts interact with water hence it is termed hydrolysis. As a result, hydrogen or hydroxyl ions remain in the solution in excess, the solution itself becoming acid or basic respectively. 

I. Salts of strong acids and strong bases, when dissolved in water, show a neutral reaction, as neither the anion nor the cation combines with hydrogen or hydroxyl ions respectively to form sparingly dissociated products. The dissociation equilibrium of water... 

For solid acids and bases, the acidic and basic properties can also be expressed by a similar equation, but it incorporates the concentrations of acidic or basic sites present on the surface instead of concentrations of hydrogen or hydroxyl ions ... 

It has been assumed hitherto that the zeta-potential acting across the solution double layer remains constant any substance which alters the zeta-potential at constant hydrogen ion concentration will, however, be expected to have some influence on the overvoltage. The effect of strongly adsorbed neutral ions or of hydrogen or hydroxyl ions at high concentrations can be largely accounted for in this manner. 

Many solution reactions are catalyzed by hydrogen or hydroxyl ions and consequently may undergo accelerated decomposition upon the addition of acids or bases. The catalysis of a reaction by hydrogen or hydroxyl ions is known as acid-base specific catalysis. In many cases, in addition to the effect of pH on reaction rate, there may be catalysis by one or more species of the buffer system. This type of catalysis is known as the acid-base general catalysis. Solutions of vitamin were found to be... 

For more complicated electrode reactions the Nemst equation includes terms to represent all species of variable activity involved in the reaction, such as hydrogen or hydroxyl ion and complexing molecules or ions. Reactants at invariant unit activity, for instance the solvent, pure metals, and pure solids, are omitted in the Nemst equation. Examples are... 

Whenever hydrogen or hydroxyl ions appear in the half-reaction, the electrode potential varies with pH. Consider the reaction... 

Hydrolysis A term used to signify reactions due to the presence of the hydrogen or hydroxyl ions of water. 

Fels was the first to take advantage of such buffer mixtures. They were frequently employed by SSrensen. In the colori-metrie determination of hydrogen ion concentration they are almost indispensable. We see that the hydrogen or hydroxyl ion concentration always lies in the neighborhood of the dissociation constant of the acid or base used. When the acid and salt concentrations are equal,... 

The sensitivity of indicator papers depends upon a number of factors which will be discussed below. Suffice it to say that this sensitivity is always smaller than that of the indicator solution when determined with strong acids or bases. When buffer mixtures are used, however, the sensitivity of the paper towards hydrogen or hydroxyl ions is the same as that of the solution of the corresponding indicator. 

The cross-links tend to make the a fold rigid and increase the amount of work needed to stretch the fibres to the /3 configuration. The effect of salt linkages is demonstrated by the relationship between work required to bring about a specified extension, and pH. Salt links exist between pH 4 and pH 8 but are ruptured by excess of hydrogen or hydroxyl ions ... 

When hydrogen ions are added to a solution, the pH decreases when they are removed (or hydroxyl ions added), the pH rises. But when certain substances are present in the solution, they will tend to combine with any H+ or OH" ions added, and, by removing them, they act so as to prevent the change in pH that would otherwise occur. Substances that act in this way are called buffers. In their ability to mop up hydrogen or hydroxyl ions, buffers act as regulators of pH. Such regulation is extremely important to the delicately balanced living cell, where sharp fluctuation in acidity and alkalinity can easily spell disaster. 

At the heart of electrolysis is an electrolyzer. An electrolyzer is a series of cells each with a positive (anode) and a negative (cathode) electrode. The electrodes are immersed in water that has been made electrically conductive by adding hydrogen or hydroxyl ions, usually in the form of highly concentrated alkaline potassium hydroxide. 

The exchange potentials of the H and OH" ions varys considerably with the nature of the functional group and depend upon the strength of the acid or base formed between the functional group and either the hydrogen or hydroxyl ion. The stronger the acid or base, the lower the exchange potential is. 

Hydrogen or hydroxyl ions are involved in many redox half-reactions. We can change the potential of these redox couples by changing the pH of the solution. Consider the As(V)/As(III) couple ... 

In aqueous solution it is not always easy to establish with certainty the existence of catalysis by species other than hydrogen or hydroxyl ions, and some of the early conclusions in this field were based on insufficient evidence. The safest method is to use as catalysts a series of buffer solutions of equal ratios but varying concentrations, using the principle of constant ionic strength to eliminate secondary salt effects, as described in the last sub-section. If the observed reaction velocity increases with increasing buffer concentration in such a series, this is proof that one or... 

This treatment is easily generalized and leads to the conclusion that in water or similar solvents general acid catalysis will be observable only for intermediate values of the exponent a. If a is too small, the catalytic effect of acids will be swamped by that of the solvent, while if a approaches unity, the effect of all other acids will be masked by that of the hydrogen ion. Similar conclusions apply to basic catalysis. It is possible, therefore, that those reactions in aqueous solution which appear to show specific catalysis by hydrogen or hydroxyl ions (cf. preceding sub-section) do not constitute a special class of reaction, but are actually... 

An exactly similar treatment can be applied to basic catalysis. The important general result of these considerations is that if general acid-base catalysis is observed in a reaction involving only one proton transfer, then this proton transfer is rate determining. However, it is not safe to assume that the converse is true, i.e., that the substrate and catalyst are effectively in equilibrium in reactions found experimentally to be specifically catalyzed by hydrogen or hydroxyl ions. This is because (as shown in Sec. II.4) catalysis by species other than H+ or OH- may frequently escape observation, giving a false impression of specific catalysis. 

The results of the above analysis of reactions involving two proton transfers may be compared with those for reactions in which only a single transfer is kinetically relevant (Sec. III.3.a). For a single transfer the existence of an equilibrium between catalyst and substrate always produces the appearance of specific catalysis by hydrogen or hydroxyl ions, and the detection of general acid-base catalysis therefore excludes... 

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